A method of this kind is known from Med. Phys. Vol. 14, No. 3, May/June 1987, pp. 330-334.
This publication describes a method of processing an X-ray image by estimating a scattered radiation field from a spatial intensity distribution of an attenuated X-ray beam. This method serves to reconstruct a corrected image having a higher contrast by subtracting a scattered radiation image from the measured intensity distribution from an original image which has been "contaminated" by a non-imaging component which becomes apparent as blurring across the image. In the absence of scattered radiation, a linear relationship exists between a logarithm of intensity and the distance in a direction of irradiation. By correction for scattered radiation the quantitative accuracy increases in densitometry where relative thickness differences are calculated from an image. A spatial intensity distribution of an X-ray beam attenuated by an object contains a component which does not contribute to imaging, inter alia because in addition to attenuation of the X-ray beam in the propagation direction also scattering of electrons from the attenuating object occurs. The intensity distribution of the scattered X-rays can be described as a convolution of a primary incident beam with a so-called point spread function. The detected intensity is taken as an estimate for the primary intensity. Any practical image of an object is spatially spread by a point spread function. Because a ratio of scattered radiation to primary radiation behind a thick object when irradiated by an X-ray beam is higher than that behind a thin object, when estimating the scattered radiation from the primary beam, the convoluted primary beam must also be weighted by a weighting factor which depends on the local transmission. The accuracy of an estimate of the scattered radiation field is inter alia dependent on the accuracy with which the local weighting factor can be determined. In the cited publication the local weighting factor is measured with a fixed setting of a tube voltage of an X-ray tube and a fixed distance between focus and detector. In a correction circuit the local weighting factors are plotted in a table. Depending on an intensity value of an element in the image matrix of the detected X-ray image, a local weighting factor is selected whereby a corresponding matrix element of the convoluted image matrix is multiplied.
A method of this kind has the drawback that the local weighting factor is applicable only to one fixed adjustment value of the imaging parameters. When a tube voltage, a tube current, a position of the X-ray focus, the patient table, a distance between the patient and the image intensifier, an active cross-section of an entrance screen of an image intensifier, etc. is changed, a new variation of the local weighting factor must be measured. Furthermore, during determination of the local weighting factor from the detected image, which may contain abrupt transition in brightness, the weighting factors may vary comparatively greatly over a short distance in the image. This causes gradients in the estimated scattered radiation image; this is a poor approximation of an actual scattered radiation image which varies only slowly as a function of location.